Bone remodeling, the process by which the adult human skeleton is continuously renewed, is carried out by osteoclasts and osteoblasts, two specialized cell types that originate from hematopoietic and mesenchymal progenitors of the bone marrow, respectively. A continuous and orderly supply of these cells is believed to be essential for skeletal homeostasis, as increased or decreased production of osteoclasts or osteoblasts and/or changes in the rate of their apoptosis are largely responsible for the imbalance between bone resorption and formation that underlies several systemic or localized bone diseases. For example, enhanced osteoclast activity has been found to play a major role in the pathogenesis of postmenopausal osteoporosis, Paget's disease, lytic bone metastases, multiple myeloma, hyperparathyroidisn, rheumatoid arthritis, periodontitis, and hypercalcemia of malignancy.
Numerous genes and gene families (and the polypeptides encoded by them) that participate in the regulation of bone cell production and apoptosis have been identified. Wnt proteins have been identified as a family of growth factors consisting of more than a dozen structurally related molecules that are involved in the regulation of fundamental biological processes such as apoptosis, embryogenesis, organogenesis, morphogenesis and tumorigenesis (Nusse and Varmus, Cell 1992, 69:1073-1087). Wnt polypeptides are multipotent factors and have biological activities similar to those of other secretory proteins such as transforming growth factor (TGF)-β, fibroblast growth factors (FGFs), nerve growth factor (NGF), and bone morphogenetic proteins (BMPs). One member of the Wnt growth factor family, termed Wnt-x, is preferentially expressed in bone tissue and in bone-derived cells and appears to be involved in maintaining the mature osteoblast (bone-forming cell) phenotype (WO 95/17416).
Studies indicate that certain Wnt proteins interact with a family of proteins named “frizzled” that act as receptors for Wnt proteins or as components of a Wnt receptor complex (in Moon et al., Cell 1997, 88:725-728; Barth et al., Curr. Opin. Cell Biol. 1997, 9:683-690). Frizzled proteins contain an amino terminal signal sequence for secretion, a cysteine-rich domain (CRD) that is thought to bind Wnt, seven putative transmembrane domains that resemble a G-protein coupled receptor, and a cytoplasmic carboxyl terminus.
The first secreted frizzled-related protein (SFRP) was named “Frzb” (for “frizzled motif in bone development”) and was purified and cloned from bovine articular cartilage extracts based on its ability to stimulate in vivo chondrogenic activity in rats (Hoang et al., J. Biol. Chem. 1996, 271:26131-26137). The human homologue of the bovine gene has also been cloned. Unlike the frizzled proteins, however, Frzb does not contain a serpentine transmembrane domain, and appears to be a secreted receptor for Wnt. The Frzb cDNA encodes a 325 amino acid/36,000 dalton protein and is predominantly expressed in the appendicular skeleton. The highest level of expression is in developing long bones and corresponds to epiphyseal chondroblasts; expression declines and disappeares toward the ossification center.
Studies indicate that SFRPs participate in apoptosis. Some SFRPs have thus been identified as “SARPs” for secreted apoptosis related proteins. Additional members of the SFRP family have been identified, and have been shown to be antagonists of Wnt action. There are currently at least five known human SFRP/SARP genes: SFRP-1/FrzA/FRP-1/SARP-2, SFRP-2/SDF-5/SARP-1, SFRP-3/Frzb-1/FrzB/Fritz, SFRP-4 and SFRP-5/SARP-3 (Leimeister et al., Mechanisms of Development 1998, 75:29-42). Secreted frizzled related protein-1 (SFRP-1) is a Wnt antagonist and is expressed in osteoblasts and osteocytes. Although the precise role that SARPs/SFRPs play in apoptosis is not yet clear, these proteins appear to either suppress or enhance the programmed cell death process. Deletion of SFRP-1 in mice has been shown to lead to decreased osteoblast/osteocyte apoptosis and to increased bone formation. (Bodine, P. V. N, et al., Mol. Endocrinol., 2004, 18(5) 1222-1237.)
A need exists in the art for the identification of inhibitors of SFRP-1 that can be used as novel agents for the treatment of bone disorders, including bone resorption disorders such as osteoporosis, and for regulation of bone formation in humans.